Keck spectra of brown dwarf candidates and a precise determination of the lithium depletion boundary in the alpha Persei open cluster

We have identified 27 candidate very low mass members of the relatively you ng alpha Persei open cluster from a 6 deg(2) CCD imaging survey. Based on t heir I magnitudes and the nominal age and distance to the cluster, these ob jects should have masses less than 0.1 M. if they are cluster members. We s ubsequently obtained intermediate-resolution spectra of 17 of these objects using the Keck II telescope and Low-Resolution Imaging Spectrograph (LRIS) . We also obtained near-IR photometry for many of the stars. Our primary go al was to determine the location of the "lithium depletion boundary" and, h ence, to derive a precise age for the cluster. Most of our program objects have radial velocities consistent with cluster membership, moderately stron g Ha emission, and spectral types of M5.5 to M8, as expected from their (R- I)(C) colors. We detected lithium with equivalent widths greater than or eq ual to 0.4 Angstrom in five of the program objects. We have constructed a c olor-magnitude diagram for the faint end of the alpha Persei main sequence, including stars for which high signal-to-noise spectra in the region of th e lithium lambda 6708 absorption line have been obtained. These data allow us to accurately determine the alpha Persei single-star lithium depletion b oundary at M(I-C) = 11.47, M-bol = 11.42, (R-I)(C0) = 2.12, spectral type M 6.0. By reference to theoretical evolutionary models, this converts fairly directly into an age for the alpha Persei cluster of 90 +/- 10 Myr. That ag e is considerably older than most previously quoted ages for the cluster bu t consistent with ages estimated from the upper main-sequence turnoff using recent models that include a moderate amount of convective core overshoot. At this age, the two faintest of our spectroscopically confirmed members s hould be substellar (i.e., brown dwarfs) according to theoretical models.